This invention relates to the storage and dissipation of heat from systems having large thermal management problems, in particular, to the storage and dissipation of energy from high power, space-based systems applications, such as microwave tubes used in weapons systems. The invention also relates to the storage and dissipation of heat from high power components of ground-based facilities, such as fuel rods in nuclear power plants.
In the operation of high energy space-based devices, thermal energy, typically generated during a burst power mode, must be dissipated and may be conveyed to an ultimate heat sink, such as outer space. It has been proposed to use a phase-change material, such as lithium salts, particularly lithium hydride, as a heat sink to remove and store excess heat during the burst power mode Thereafter, the heat may be dissipated from the heat sink over a longer period of time to an ultimate heat sink. Phase-change materials have a high heat of fusion which enables the storage of significant amounts of thermal energy as such materials change from solid to liquid Phase. They later resolidify as the thermal energy is dissipated to an ultimate heat sink.
While lithium hydride has high thermal energy storage capacity, it has very low thermal conductivity. Problems exist, therefore, in conducting thermal energy into phase-change materials for storage. Further, because of low thermal conductivity, heat sink surfaces closest to heat sources develop excessive temperatures.
In an attempt to use the desirable thermal storage capacity of phase-change materials, solutions to management problems presented by large or high-power systems have been proposed, wherein phase-change materials, are encapsulated in shells, preferably cylinders or spheres, which are then submerged in baths of high thermal conductivity materials. Typically in applications such as described above, the heat sink is designed to surround the heat source. While submerging shells of phase-change materials in such baths improves the overall thermal conductivity of the heat sink and facilitates the storage of energy, problems remain with excessive temperatures occurring on surfaces of the heat sinks.
One approach to reduce the excessive surface temperatures is to further increase the overall thermal conductivity of the heat sink so that the thermal management systems may even more effectively conduct the heat away from the source. Conventional means for increasing system thermal conductivity, such as increasing the size or amount of thermally conductive bath materials, are unacceptable due to size and weight constraints on space-based systems and size limitations on ground-based systems applications.
Another approach is shown by Kennel, U.S. Pat. No. 4,755,350 where the heat source surrounds the heat sink of a thermal management system. A phase-change material is used to absorb heat from thermionic emitter electrodes for space-based weapons applications. A single conventional heat pipe located along the central axis of the phase-change material carries away waste heat stored in the phase-change material. This approach, however, is limited to applications where phase-change materials are enclosed or surrounded by heat sources, and the size of the heat sink is limited by the physical dimensions of the heat source present in a particular application. Moreover, in some applications, such as the space and ground based applications described above, a single conventional high capacity heat pipe such as shown by Kennel is not capable of transferring thermal energy from the source at desired rates.
There remains, therefore, a need to provide more effective, low thermal resistance heat sinks wherein thermal energy from heat sources may initially be stored in phase-change materials and later dissipated to an ultimate heat sink.